1. Field of the Invention
The present invention relates to a zoom lens system and, in particular, to a zoom lens system suitable for an imaging lens system employed in an interchangeable lens apparatus in a so-called interchangeable-lens type digital camera system (simply referred to as a “camera system”, in some cases hereinafter). Further, the present invention relates to an interchangeable lens apparatus and a camera system that employ this zoom lens system.
2. Description of the Background Art
The market is rapidly growing for interchangeable-lens type digital camera systems each including: a camera body employing an image sensor composed of a CCD (Charge Coupled Device), a CMOS (Complementary Metal-Oxide Semiconductor) or the like; and an interchangeable lens apparatus employing an imaging lens system for forming an optical image on the light acceptance surface of the image sensor, wherein the imaging lens system is attachable to and detachable from the camera body. As for the interchangeable lens apparatuses, those employing a zoom lens system capable of forming an optical image with variable magnification are widely favored.
Among zoom lens systems, in particular, telephoto-oriented zoom lens systems have a long focal length at a telephoto limit, and hence often have a long overall optical length (a distance from the top of the lens surface on the most object side to the image surface). Thus, such a configuration is often employed that a lens unit having positive optical power is arranged on the most object side and a lens unit having negative optical power is arranged on the most image side so that the overall optical length at a telephoto limit is made shorter than the focal length at a telephoto limit.
In telephoto-oriented zoom lens systems, configurations have been proposed that the number of lens units is increased for the purpose of reducing various kinds of aberration. For example, a zoom lens system consisting of five lens units of positive, negative, negative, positive and negative has been proposed (e.g., Japanese Patent Publication No. 3134448 (Reference 1)). Further, in telephoto-oriented zoom lens systems, the long focal length at a telephoto limit easily enhances image blur in association with vibration. Thus, methods have been proposed that a part of lens units (an image blur compensation lens unit) is parallel-displaced in a direction perpendicular to the optical axis in accordance with a change in the orientation of the entire lens system (e.g., Japanese Laid-Open Patent Publication No. H6-123836 (Reference 2), Japanese Patent Publication No. 3395169 (Reference 3), Japanese Laid-Open Patent Publication No. H6-130330 (Reference 4) and Japanese Laid-Open Patent Publication No. H11-202201 (Reference 5)).
Each of the telephoto zoom lens systems described in References 2, 3, and 4 consists of five lens units of positive, negative, negative, positive and negative in order from the object side to the image side. The telephoto zoom lens system described in Reference 5 consists of five lens units of positive, negative, negative, positive and negative or alternatively positive, negative, positive, positive and negative in order from the object side to the image side. In each telephoto zoom lens system, one of the five lens units is parallel-displaced in a direction perpendicular to the optical axis so that image blur compensation is achieved. When the individual lens units are referred to as the first lens unit, the second lens unit, . . . , and the fifth lens unit in order from the object side to the image side, the third lens unit in the telephoto zoom lens system described in Reference 2 is parallel-displaced in a direction perpendicular to the optical axis. Similarly, the second lens unit in the telephoto zoom lens system described in Reference 3, the fourth lens unit of the telephoto zoom lens system described in Reference 4, and a part of the second lens unit of the telephoto zoom lens system described in Reference 5 are parallel-displaced in a direction perpendicular to the optical axis.
In focusing in a telephoto zoom lens, a method of moving the first lens unit located on the most object side has widely been employed. Nevertheless, this focusing method based on the movement of the first lens unit causes a problem that high-speed auto-focusing cannot be achieved because of the largeness and heaviness of the first lens unit. For the purpose of resolving this problem, in the telephoto zoom lens system described in Reference 5, focusing from an infinite distance to a close distance is proposed to be achieved by moving the fourth lens unit in an optical axis direction.
In interchangeable-lens type digital camera systems, video image taking is also desired in addition to still image taking. However, in video image taking, auto-focusing need be performed continuously at a high speed.
In order that auto-focusing should be performed continuously at a high speed, for example, a method of repeating a series of the following operations: oscillating (wobbling) a part of lens units in the optical axis directions at a high speed so that a situation of “out-of-focus state->in-focus state->out-of-focus state” is obtained; detecting, from the output signal of the image sensor, signal components in a predetermined frequency band in a part of the image region so that an optimal position realizing the in-focus state is calculated for the focusing lens unit; and moving the focusing lens unit to the optimal position, may be adopted. In order that uneasiness such as flickers should be avoided, video displaying need be performed at a high rate of, for example, 30 frames per second. Thus, basically, image taking also need be performed at the same rate of 30 frames per second. Accordingly, in auto-focusing in video image taking, the focusing lens unit need be wobbled continuously at the high rate of 30 Hz.
When the above-mentioned wobbling is to be employed, it should be noted that the size of the image corresponding to a photographic object varies in association with wobbling. This variation is caused mainly by the fact that the movement of the focusing lens unit in the optical axis directions generates a change in the focal length of the entire lens system. Then, when a large change in the image taking magnification is generated in association with wobbling, uneasiness is caused.
When the above-mentioned facts are taken into consideration, in order that the outer diameter of the lens barrel should be reduced, the weight of the image blur compensation lens unit and the focusing lens unit need be reduced as light as possible. For this purpose, the outer diameters of the individual lens elements that constitute the image blur compensation lens unit and the focusing lens unit need be reduced as small as possible so that the weight of the individual lens units need be reduced as light as possible. In this view, the techniques in the above-mentioned references have problems individually.
For example, References 1 to 4 do not mention image blur compensation. Further, in the zoom lens system described in Reference 5, the fourth lens unit that moves at the time of focusing is also composed of three or four lens elements. Thus, the fourth lens unit is heavy. This causes a problem that a larger motor or actuator for moving the fourth lens unit is required.